If you are going to do this, please ensure that you have full backups before trying to move the folder to another partition. I cannot say the results as I've never tried it previously. If you have full backups, you can at least restore the account or folder if needed.
I did do some basic testing doing the steps below by moving the mail folder for a testing domain and then symlinking it back. After performing those steps, I was still able to create new email accounts, log into webmail interface and send a test email from Roundcube. So those areas appear to still function per the very quick test I performed.
For moving it, all you'd do is the following (please do not create the email folder beforehand and let it be created with the move command):
A move normally maintains the permissions, so you should then be able to do the symlink:
mv /home/username/mail /usr/username-mail
Please replace username in all instances to the cPanel account username.
ln -s /usr/username-mail mail